A certain number of service companies have been measuring for a long time the gas content of mud (mud logging—measurement of the C1–C5 constituents) and some of these companies currently develop measuring tools allowing to quantify more precisely the gas content of mud, notably for the hydrocarbons of C6–C8 fractions.
In exploration as well as in development, when a hydrocarbon-rich level is discovered during drilling, it is very important for the operator to know the type of fluids present in the formation (oil, gas, condensate, heavy oil, etc.) or, even better, the volume ratio of gas to oil in the fluids of a well. This information is currently available only by means of formation tests, which requires drilling stop, as described in for example U.S. Pat. Nos. 5,635,631 and 5,612,493, and represents a considerable cost for the operator. It is therefore clear that an approximation of this GOR during drilling would be an important “plus” for the operator because this information would allow to better apprehend the economical interest of the potential deposit and would lead to a better evaluation of the rest of the drilling operation (drilling stop or continuation, absence or start of production tests).
In the present state of our knowledge, there is no method allowing to know this GOR without carrying out formation tests. There are only more or less empirical methods based on molecular ratios of the gases from the drilling mud whose interpretation allows, at best, to know if the fluid present in the reservoir is oil, gas or heavy oils.